The power from a nuclear reactor is controlled using nuclear reactor control rods, the insides of which are filled with a material that absorbs neutrons, with the control rods being inserted into the reactor core or retracted from the reactor core as required (see PTL 1).
These control rods are stored inside control rod cluster guide tubes provided above the nuclear reactor vessel, and are moved up and down using a control rod drive device positioned above the guide tubes. These control rod cluster guide tubes each comprise, for example, an upper guide tube with a circular cross-section and a lower guide tube with a square cross-section, with these upper and lower guide tubes forming the external shape of the guide tube.
The control rod cluster guide tubes are installed in a region corresponding with a high flow rate for the main flow of the primary coolant, and the upper guide tube and lower guide tube perform the role of protecting the control rods from the fast flowing primary coolant in order to enable the control rods to be positioned accurately to facilitate their insertion and retraction. The lower guide tube is sometimes referred to as an enclosure because it encloses the periphery of the control rod.
In the control rod cluster guide tubes, a flange fitted to the bottom end of the upper guide tube is abutted against a flange fitted to the top end of the lower guide tube, and bolts are then used to secure the guide tube to the upper surface of an upper core support plate. The flange fitted to the top end of the lower guide tube is positioned near the middle of the control rod cluster guide tube in the height direction, and is therefore sometimes referred to as the middle flange.
Conventionally, attachment of the flange to the lower guide tube is performed by inserting the lower guide tube within a through-hole formed in the flange, and then fillet welding the outer peripheral surface of the lower guide tube to the flange surface, and the tip of the lower guide tube to the inner peripheral surface of the through-hole.